Weighing devices may be used as quality control tools in a manufacturing facility, for example in a government mint, where the accurate weight of coins is of concern for several reasons. In particular if the coins are of a precious metal such as gold or silver, underweight coins would be objectionable to the public as representing less than full value, while overweight coins would be considered lost profit (commonly referred to as give-away) by the manufacturer. General-circulation coins, too, have to conform to specified weight tolerance limits, even though they are generally made of inexpensive alloys and/or laminates and their material value is usually lower than the face value of the coin, since out-of-tolerance coins could fail to work in coin-operated devices of any kind, such as parking meters, coin washers, vending machines, automated supermarket terminals and the like. Also, inconsistent coin weights could cause miscounts on counting-by-weight scales.
The weighing of mass-produced items in a production line is a well-known quality control function in many industries, for example to ensure the correct fill weight of packages in a food-processing plant. The standard solution for this kind of application is a so-called weighing conveyor or conveyor belt scale as described and illustrated, for example, in U.S. Pat. No. 4,440,249. A compact and self-contained belt conveyor is supported by one or more weighing cells which, in turn, are mounted on a support frame. This entire assembly is placed inline between an incoming and an outgoing transport conveyor, also referred to as infeed conveyor and outfeed conveyor. The weight of every item traveling down the production line is thus being checked while the item is moving over the weighing conveyor.
For reasons that are inherent in its working principle, this otherwise widely used kind of inline conveyor scale could not be considered for the task of checking the weights of smaller items such as, for instance, coins. Primarily, such a conveyor scale would be totally outside its realistic performance range when attempting to measure the small weights of bullion coins (typically in the range of 0.1 to 1.0 ozt, i.e., 3.1 to 31 grams, with a 2σ-precision of ±2 mg to ±5 mg), considering in particular, that this weighing precision would have to be achieved on top of a large dead load represented by the belt conveyor, and furthermore in the presence of vibrations from the motor and moving parts of the conveyor.
On the other hand, the precise weighing of coins in the aforementioned weight range is a simple routine task when performed manually on a commercially available laboratory balance displaying weight in grams to three or four decimal places. To the applicant's knowledge, this is in fact how freshly minted bullion coins or medals are being checked in government or commercial mint facilities. Although the aforementioned weighing precision can easily be met or even exceeded, the manual weighing of individual coins on a laboratory balance is a tedious, labor-and cost-intensive activity, and therefore a prime candidate for automation.